1. Field of the Invention
The present invention relates to a sheet conveyance apparatus with which image forming apparatus such as copiers, printers and facsimile machines and image reading apparatus such as scanners are equipped.
2. Description of the Related Art
Recently, in image forming apparatus, together with a request for high productivity (a number of sheets on which an image can be formed per unit time) and miniaturization of devices, a request that skew and displacement of a sheet is corrected with high precision to improve image quality, is also increasing. Further, sheets to be conveyed in sheet conveyance apparatus have various thickness from heavy paper to thin paper, various sizes from a small size of a post-card size to a large size such as 330 mm×488 mm, and have various paper quality such as coated paper and embossed paper, in addition to plain paper.
In order to achieve high productivity, firstly, a distance between conveyed sheets (a distance between the rear edge of the preceding sheet and the leading edge of the following sheet) needs to be reduced as much as possible. When the distance between the sheets is reduced, skew and displacement that occur in feeding a sheet have to be corrected in a short time.
Therefore, a method of correcting the skew while a sheet is conveyed has been proposed. The method uses a skew correction unit for correcting the skew of a sheet instead of a conventional method in which skew is corrected by abutting the leading edge of a sheet onto a nip of a stopped roller pair. This technique is disclosed in Japanese Patent Application Laid-Open No. 4-277151.
The skew correction method is a so-called active registration method, and has, for example, a configuration as shown in FIG. 16 in which two sensors 500a and 500b are arranged in a direction (sheet width direction) orthogonal to a sheet conveyance direction (a sheet moves from the left to the right in the figure), and the sensors 500a and 500b detect the leading edge of the conveyed sheet S. Then, a skew amount of the leading edge of the sheet S is calculated based on a signal detected when the sheet S passes the sensors 500a and 500b. Thereafter, the skew of the sheet S is corrected according to the skew amount calculated by a skew correction roller pair 504a and 504b. The skew correction roller pair 504a and 504b is arranged on the same axis in the sheet width direction at a predetermined interval, and drive of the roller pair 504a and 504b is independently controlled by motors 502a and 502b. In this way, the skew can be corrected even if a distance between sheets is small.
As a conventional method of correcting displacement in the sheet width direction, a configuration is proposed in which a registration roller pair is moved in a thrust direction to correct the lateral registration position of a sheet. This technique is described in Japanese Patent Application Laid-Open Nos. 59-4552 and 3-94275.
An image forming unit is normally arranged downstream from a registration roller pair. A sheet held between the registration roller pair is moved in the thrust direction to correct the lateral registration position by the time the sheet is conveyed to the image forming unit. In this case, when the rear edge of the sheet passes through the registration rollers, the registration rollers must be returned to their initial positions to prepare for the following sheet. If the distance between conveyed sheets is small, control is performed such that the registration roller pair conveys the sheet to the image forming unit, the registration roller pair is released from a nip after the leading edge of the sheet reaches the image forming unit, and the registration rollers are returned to their initial positions. In this way, lateral registration can be corrected even if a distance between sheets is small.
On the other hand, various basis weight of a sheet is required, which ranges from thin paper of about 50 g/m2 to thick paper of 300 g/m2 or more. The sheet size is also diversified into various kinds from a small size such as a post-card size to a large size such as 330 mm×488 mm. In order to correct the skew and displacement of a large, thick sheet having large inertia force with high precision, a conveyance load on a sheet needs to be reduced as much as possible. Typically, all conveyance rollers upstream from the skew correction roller pair 504a and 504b shown in FIG. 17 are released from a sheet and a conveyance guide is configured to be a straight path so that a conveyance load on a sheet is reduced.
However, a straight conveyance guide in a registration unit increases a size of the entire device. Accordingly, a bending conveyance guide needs to be arranged upstream of a registration unit in order to decrease the size of a device. In this case, if the skew correction and the lateral registration movement are preformed on a large, thick sheet having large inertia force and large bending rigidity, a large conveyance resistance is applied to the sheet from the bending conveyance guide. Therefore, sheet slippage occurs when skew correction is performed on the sheet so that precision in correcting skew and lateral registration deteriorates.